Satellite communication systems typically have employed large aperture antennas and high power transmitters for establishing an uplink to the satellite. Recently, however, very small aperture antenna ground terminals, referred to as remote ground terminals, have been developed for data transmission. In such systems, the remote ground terminals, also known as VSATs, are utilized for communicating via a satellite from a remote location to a central hub station. The central hub station communicates with multiple remote ground terminals, and has a significantly larger antenna, as well as a significantly larger power output capability than any of the remote ground terminals.
As stated, very small aperture terminals (VSATs) can be used to communicate data, voice and video, to or from a remote site to a central hub. Typically, the VSAT terminals have a small aperture directional antenna for receiving from or transmitting signals to the satellite, and an outdoor unit (ODU) mounted near the antenna for transmitting a modulated carrier generated by an indoor unit (IDU). The IDU demodulates incoming signals received from the ODU and also operates as an interface between a user""s communication equipment and the ODU.
The viability of the remote ground terminal concept increases as the cost for providing a remote ground terminal at the remote location decreases. In pursuit of this objective, various techniques have been utilized to reduced the cost of the remote ground terminal. For example, U.S. Pat. No. 5,208,829 describes a spread spectrum technique that maximizes the power output from a satellite so as to allow a cost saving reduction in the size of the antenna of the remote ground terminal. U.S. Pat. No. 5,309,479 describes a remote ground terminal comprising a low cost transmitter for producing an FSK (frequency-shift keying) modulated uplink signal which is utilized to communicate with the central hub station via the satellite.
Notwithstanding these cost saving techniques, problems remain in prior art VSAT systems. For example, in prior art systems, which modulate an intermediate frequency signal and then up-convert the signal to a carrier frequency, the frequency deviation or error is also multiplied by the same factor as the frequency, which can be on the order of 100 or greater. As a result, the frequency of the modulated carrier signal is susceptible to undesirable variations.
Furthermore, typically, the outdoor unit contains circuitry for generating a vector modulated data signal, including linear upconverters for frequency converting the modulated data signal to the required transmit frequency and a linear power amplifier for amplifying the modulated data signal prior to transmission via the antenna. Such circuitry presents numerous drawbacks, for example, the generation of spurious signals in the outdoor unit which operate to increase the noise component of the output signal transmitted to the satellite.
Accordingly, there exists the need for a VSAT system comprising an indoor unit and an outdoor unit, which eliminates the foregoing problems.
The present invention relates to a VSAT system comprising both an indoor unit and an outdoor unit, which solves the foregoing problems. More specifically, the VSAT system of the present invention comprises an indoor unit having a novel synthesizer/modulator design which allows for the modulation and frequency multiplication of an intermediate frequency signal without a corresponding increase in the frequency deviation or phase noise of the signal. In addition, the present invention provides a novel design for the outdoor unit which eliminates the need for the transmitter module to contain a linear upconverter or a linear power amplifier, so as to substantially reduce the overall cost of the remote ground terminal relative to prior art designs. The design of the transmitter module also minimizes the outdoor unit""s sensitivity to changes in temperature.
In an exemplary embodiment the indoor unit comprises a vector modulator for phase-modulating an input information signal so as to produce a phase-modulated signal; a first frequency divider circuit coupled to the output of the vector modulator, which is operative for reducing the frequency and frequency deviation of the phase-modulated signal, and for generating an output signal having an envelope of constant amplitude; and a mixer coupled to the first frequency divider, which is operative for frequency translating the phase-modulated signal and outputting a modulated data signal. The outdoor unit comprises a phase-locked loop coupled to a transmitter module. The phase-locked loop and the transmitter module operate to receive the modulated data signal and to frequency multiply and amplify the modulated data signal so as to produce a modulated carrier signal having an envelope of constant amplitude.
As described below, the VSAT system of the present invention provides important advantages over prior art devices. For example, by downconverting the modulated reference signal output by the modulator, the frequency deviation of the modulated reference signal is also reduced by the same factor. Accordingly, inexpensive modulators operating in the microwave region can be utilized. Without such down-conversion, upon completing the upconversion process, the frequency deviation of the modulated reference signal would exceed the frequency deviation rating of the modulator.
Furthermore, the down-conversion of the modulated reference signal also provides the added benefit that any error in the I/Q balance of the input data signals to the modulator is also reduced by the same factor.
In addition, the design of the transmitter module of the present invention results in an increase in operational efficiency of the outdoor unit, a reduction in overall size of the outdoor unit and a reduction in DC power consumption by the outdoor unit, which substantially lowers the cost of the outdoor unit of the present invention.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.